1. Field of the Invention
The present invention relates to a light emitting device prepared with a light emitting element capable of obtaining fluorescence or phosphorescence. In particular, the present invention relates to a light emitting device in which an active element such as an insulated gate transistor or a thin film transistor, and a light emitting element connected to the active element, are formed in each pixel.
2. Description of the Related Art
Display devices using liquid crystals are structures that typically employ a back light or a front light, and display an image by using light from the back light or the front light. Liquid crystal display devices are employed as image displaying means in various types of electronic devices, but have a structural disadvantage in that they possess a small angle of view. In contrast, the angle of view is wide for display devices that use a light emitter capable of obtaining electroluminescence as displaying means, and there is superior visibility. Such devices are therefore looked upon as next generation display devices.
The structure of a light emitting element which uses an organic compound as a light emitter (hereafter referred to as organic light emitting element) is one in which hole injecting layers, hole transporting layers, light emitting layers, electron transporting layers, electron injecting layers, and the like formed by organic compounds are suitably combined between a cathode and an electrode. Hole injecting layers and hole transporting layers are differentiated and denoted separately here, but they are the same in that their hole transporting characteristics (hole mobility) are very important in particular. For convenience in making the distinction, the hole injecting layer is the layer contacting the anode, and the layer contacting the light emitting layer is referred to as the hole transporting layer. Further, the layer contacting the cathode is referred to as the electron injecting layer, and the layer contacting the other side of the light emitting layer is referred to as the electron transporting layer. The light emitting layer may also serve as the electron transporting layer, and is referred to as a light emitting electron transporting layer in that case.
A mechanism in which light is emitted by electroluminescence can be considered as a phenomenon in which electrons injected from a cathode and holes injected from an anode recombine in a layer made from a light emitter (light emitting layer), forming excitons. Light is emitted when the excitons return to a base state. Fluorescence and phosphorescence exist as types of electroluminescence, and these can be understood as light emission from singlet state excitation (fluorescence) and light emission from triplet state excitation (phosphorescence). The brightness of the emitted light extends from several thousands to several tens of thousands of cd/m2, and therefore it can be considered possible in principle to apply this light emission to display devices and the like. However, problems remain in that degradation phenomena exist, and this impedes utilization of electroluminescence.
Low molecular weight organic compounds and high molecular weight organic compounds are known as organic compounds for forming organic light emitting elements. Examples of low molecular weight organic compounds include: α-NPD (4,4′-bis-[N-(naphthyl)-N-phenyl-amino] biphenyl) and MTDATA (4,4′,4″-tris-(N-3-methylphenyl-N-phenyl-amino) triphenyl amine), both copper phthalocyanine (CuPc) aromatic amine-based materials, used as hole injecting layers; and a tris-8-quinolinolate aluminum complex (Alq3) and the like used as light emitting layers. Among high molecular weight organic light emitting materials, polyaniline, polythiophene derivatives (PEDOT), and the like are known.
From the standpoint of material diversity, low molecular weight organic compounds manufactured by evaporation have striking diversity compared with high molecular weight organic materials. However, whichever type is used, organic compounds made purely from only basic structural units are rare, and there are also times when dissimilar bonds and impurities are mixed in during manufacturing processes, and times when various additives such as pigments are added. Further, there are materials which deteriorate due to moisture, materials which easily oxidize, and the like among these materials. Moisture, oxygen, and the like can easily mix in from the atmosphere, and therefore it is necessary to exert care in handling.
JP 8-241047 A may be referred to for an example of a combination of a thin film transistor (TFT) and a light emitting element. This publication discloses a structure in which an organic electroluminescence layer is formed, through an insulating film made from silicon dioxide, on a TFT that uses polycrystalline silicon. Further, a passivation layer, having an end portion which is processed into a tapered shape on an anode, is positioned in a lower layer side of the organic electroluminescence layer. A material having a work function equal to or lower than 4 eV is selected for a cathode. A material in which a metal such as silver or aluminum is alloyed with magnesium is applied.
Impurities form localized levels within a forbidden band caused by oxygen in semiconductor elements having semiconductor junctions such as diodes, and the localized levels become a factor in junction leakage and reducing the carrier lifetime, and are known to greatly lower the characteristics of the semiconductor elements.
Six types of factors can be considered for organic light emitting element degradation: (1) chemical change of organic compounds; (2) organic compound melting due to heat generated during driving; (3) dielectric breakdown originating in macro defects; (4) electrode degradation or electrode and organic compound layer interface degradation; (5) degradation caused by instabilities in organic compound amorphous structure; and (6) irreversible damage due to stress or distortion caused by the element structure.
The factor (1) above has its origin in chemical change when passing through an activation state, chemical change due to certain gasses, or water vapor, which are corrosive to organic compounds, or the like. The factors (2) and (3) result from degradation caused by driving the organic light emitting elements. Heat generation is inevitable due to electric current within the elements being converted to Joule heat. Melting occurs if the melting point, or the glass transition temperature, of the organic compound is low, and electric fields concentrate in portions where pinholes or cracks exist, causing dielectric breakdown. Degradation advances even at the room temperature due to the factors (4) and (5). The factor (4) is known as dark spots, and originates in cathode oxidation and reactions with moisture. Regarding the factor (5), all organic compounds used in the organic light emitting elements are amorphous materials, and crystallize due to heat emission when stored over a long period of time or are aged, and it can be thought that almost none are capable of stably preserving their amorphous structure. Further, irregularities such as film cracking and fracture due to distortion generated by a difference in the thermal expansion coefficient of structural materials leads to the factor (6). In addition, progressive irregularities such as dark spots also are generated in those portions.
Dark spots are suppressed considerably by raising the level of a sealing technique used, but actual degradation is generated by a combination of the aforementioned factors, and it is difficult to prevent. A method of sealing organic light emitting elements formed on a substrate by using a sealing material, and forming a drying agent such as barium oxide in the sealed space has been devised as a typical sealing technique.
It is known that organic compounds form double bonds due to photo-deterioration, changing into structures containing oxygen (such as —OH, —OOH, >C═O, —COOH). It can therefore be thought that the bonding state changes, and degradation proceeds, for cases in which the organic compounds are placed within an atmosphere containing oxygen, or for cases in which oxygen and moisture are contained within the organic compounds as impurities.
FIG. 17 is a graph showing the distribution in the depth direction of oxygen (O), nitrogen (N), hydrogen (H), silicon (Si), and copper (Cu) in an organic light emitting element as measured by secondary ion mass spectroscopy (SIMS). The structure of a sample used in the measurement is as follows: a tris-8-quinolinolate aluminum complex (Alq3)/a carbazole-based material (Ir(ppy)3+CBP)/copper phthalocyanine (CuPc)/a conductive oxide material (ITO)/a glass substrate. As shown by the chemical formula in FIG. 18B, Alq3 contains oxygen within its molecules.
On the other hand, the structures shown in the chemical formulae in FIG. 18C and FIG. 18A for (Ir(ppy)3+CBP) and CuPc do not contain oxygen within their molecules.
The highest occupied molecular orbital (HOMO) degenerates, and therefore oxygen molecules are triplet state specific molecules at a base state. An excitation process from a triplet to a singlet normally becomes a forbidden transition (spin forbidden), and thus difficult to occur. Singlet state oxygen molecules therefore do not develop. However, if triplet state excitation state molecules (3M*) having a higher energy state than that for singlet excitation exist in the periphery of the oxygen molecules, then an energy transfer like that shown below occurs, and this can lead to a reaction in which singlet state oxygen molecules develop.
It is said that 75% of the molecular excitation states in the light emitting layer of an organic light emitting element are triplet states. Singlet state oxygen molecules can therefore be generated by the energy transfer of FIG. 19 for cases in which oxygen molecules are mixed inside the organic light emitting element. Singlet excitation state oxygen molecules have ionic qualities (electric charge polarization), and therefore the possibility of a reaction with charge polarization that develops in the organic compound can be considered.
For example, methyl groups in vasocuproin (hereinafter, referred to as BCP) are electron donators, and therefore carbon bonded directly to conjugate rings is charged positively. Singlet oxygen having ionic qualities reacts as shown in FIG. 18D if there is positively charged oxygen, and there is a possibility that carboxylic acid and water are formed as shown in FIG. 18E. As a result, it is expected that the electron transporting characteristics become deteriorated.
On the other hand, TFTs using a semiconductor as an active layer are damaged by alkaline metals and alkaline earth metals, which are used as cathode materials in organic light emitting elements. That is, mobile ions in these materials get mixed into a gate insulating film or within an active layer, and switching operations thus become impossible. In semiconductor manufacturing processes, it is necessary to reduce the concentration of these metallic impurities to be equal to or less than 109 atoms/cm2.